In general, a metal oxide semiconductor gas sensor is a sensor for sensing a gas from an object in a way of measuring a change in electrical conductivity occurring when a metal oxide semiconductor reacts with the gas.
Such a metal oxide semiconductor gas sensor can be inexpensively manufactured in a small size, and also has a number of advantages, such as a high stability at high temperature as well as high sensitivity and fast response speed, so that the metal oxide semiconductor gas sensor is widely used.
In particular, in the case of using a metal oxide semiconductor having a nanostructure, it is possible to maximize a reaction between the metal oxide semiconductor and the gas, due to nanostructure's inherent characteristics of a large specific surface area, due to characteristics of an electron path being several tens of nm or less when compared to a depletion layer of a sensor surface being several tens of nm or less, and due to characteristics of a gas being able to be rapidly diffused into the metal oxide semiconductor having a nanostructure having a large porosity, and thus to further enhance the above-described advantages of the metal oxide semiconductor gas sensor.
However, a gas sensing layer including the metal oxide semiconductor needs to be interposed between a pair of electrodes because the metal oxide semiconductor should sense electrical conductivity. This however brings about problems in that effective contact between the gas and the gas sensing layer is interrupted, resulting in degradation of characteristics such as sensitivity and response speed.
Meanwhile, a method of growing a nano-wire using chemical vapor deposition, a method of preparing a sensing layer using a colloidal template, a method of forming a nano-fiber layer using an electro-spinning, a method of forming a surface nanostructure using a laser, a surface etching method using anodized aluminum oxide or the like, has been used as a conventional method for forming such a nanostructure.
However, such methods for forming a nanostructure have major downsides in terms of production costs, simplicity of the method, integrability and arrayablity.
First, when a nanostructure is formed by chemical vapor deposition, reproducibility is significantly reduced. Especially, in the case of a nano-wire, it is difficult to constantly control a thickness, length, growth direction, etc., of the nano-wire in each process, thus making it difficult to manufacture a device having the same performance even through the same process. Furthermore, the chemical vapor deposition for forming a nanostructure includes a high-temperature process and a chemical reaction, thus making it difficult to integrate and array the sensor.
Also, a method of forming a nanostructure by forming a nano-pattern and etching a surface by using a colloidal template, anodized aluminum oxide, an electron beam lithography, a nano-imprint, a nano-sphere, etc., provides disadvantages such as complicated process steps and high production costs.
In addition, the method is difficult to achieve a large-sized device and is not a process of manufacturing a general semiconductor micro device. Thus, there may be a problem in compatibility with other device fabrication processes, such as an electronic nose fabrication.
Furthermore, a method for forming a nanostructure on a surface by using a laser has a relatively simple process step, but a specific surface area the nanostructure is not greater than those obtained by other methods. Moreover, it may be difficult to control a shape of the nanostructure, and it may be problematic in bond formation by an exposure to laser radiation and compatibility with other device fabrication.
To overcome these disadvantages, there is a method of forming a nanostructure by using oblique angle deposition. The oblique angle deposition, which is a method of performing deposition while a substrate makes a certain oblique angle with a flux direction of a deposition material, enables a high-porosity thin film having a nanostructure to be formed, due to a selfshadowing effect caused by a surface diffusion of a deposition material and an initial deposition material.
In addition, the oblique angle deposition may use physical vapor depositions, such as electron beam evaporation, sputter deposition, and pulsed laser deposition, which are widely used in a semiconductor micro device fabrication, and is thus very suitable for miniaturization, integration, simplification in arraying, and enlargement of the gas sensor device.
A humidity sensor with a nanostructure manufactured by the oblique angle deposition has been previously reported. Since the humidity sensor uses a change in an optical property of an oxide thin film, it is possible to fully make use of an advantage of a nanostructure formed by the oblique angle deposition with only a substrate disposed under the thin film.
A gas sensor for sensing a reducing or oxidizing gas should measure a change in electrical conductivity of a metal oxide thin film but cannot measure the change in electrical conductivity with an existing configuration on the substrate. Therefore, upper and lower electrodes need to be provided with a metal oxide thin film interposed therebetween. However, in this case, a gas and a metal oxide thin film as a gas sensing layer are not efficiently brought into contact with each other, thus making it difficult to realize performance of the gas sensor.
To explain in more detail, since a metal oxide thin film formed using the oblique angle deposition has a nanostructure array vertically arranged, electrical conductivity in a direction parallel to a substrate is difficult to be obtained, so that a change in conductivity between lower electrodes is difficult to be measured without an upper electrode.
Furthermore, when the gas sensor is configured with only the lower electrodes without the upper electrode, it has a nanostructure having a moving path of an electron between the lower electrodes. In this case, a contact area between the substrate and the nanostructures increases, and it is thus undesirable to measure a minute change in electrical conductivity.
Accordingly, the upper electrode is necessary. Here, the upper electrode should connect every upper portion of the nanostructure array, and should prevent a short with the lower electrode caused by deposition of an electrode material on a side surface of the nanostructure. This is because an electron path does not form through the metal oxide semiconductor, but through the electrode material.
In order to form the upper electrode, complex processes should be performed such as formation of a nanostructure, coating of a resist, exposure of the nanostructure by etching, deposition of an electrode, additional removal of the resist or the like. This is not an easy way in terms of cost and process simplification, and also not a desirable method in terms of compatibility with a semiconductor micro device manufacturing process since coating and etching, etc., should be performed.
Because of such difficulties, there has been no case of applying a metal oxide having a nanostructure manufactured by using an oblique angle deposition having the above-mentioned advantages, to a gas sensor of sensing a reducing gas and an oxidizing gas.